Multilevel converters/inverters, including neutral point clamped (NPC), flying capacitor (FLC), cascaded H-bridge (CHB) and modular multilevel converters/inverters, have drawn attention in recent years, especially in medium and high voltage/power applications, such as motor drive systems, traction, PV inverters and battery charging stations. Their advantages include high reliability, low voltage stress, low electromagnetic interference (EMI) and low common-mode voltages. Pulse width modulation (PWM) modulation techniques, including selective harmonic elimination (SHE) and selective harmonic compensation (SHC), have been applied in multilevel converters/inverters to achieve high performance with low switching frequencies in various applications such as static synchronous compensators and active power filters (APF). In these techniques, the transcendental equations to be solved are developed based on voltage/current references, total harmonic distortion (THD) requirements and other objectives with the help of Fourier transformation.
Techniques including iterative numerical algorithms, online calculations and the complete solution have been proposed to solve the transcendental equations. However, the effective modulation index range of these techniques has been shown to be very narrow and their applications limited. Other techniques have been used to extend modulation ranges, but have been shown to have increased switching frequency and switching power loss, and have difficulty meeting harmonic requirements.